Definitely as soon as you can, get a helmet with a socket and put the highest level ruby you can, it adds +%exp.
It's easiest if you have a leveled up friend who can powerlevel you by rushing through everything, or if you're funded you can get spectrum (or some high dmg low-level weapon) + cains set (3 pc = +30% exp) + ruby helm socket for crazy bonus exp and crazy early damage, this build can be used on all your new classes, so it's worth investing in them right from the start if you can.
I've been rushing Act I, II, III & IV all on MP10 right from the start with my first DH today, but like I said, you wanna get funded first. Look in the black market for some gold or powerleveling services; I spent like $5 worth of gold on someone's service in the black market to get my 60 wizard geared up to 140k DPS unbuffed from what was originally less than 20k. I still have millions of gold left over from just that.